Protective circuit



April 2, 1968 R. WEINGER PROTECTIVE CIRCUIT Filed May 12, 1967 2Sheets-Sheet l INVENTOR.

K BY April 1968 R. WEINGER 3,376,477

PROTECTIVE CIRCUIT Filed May 12, 1967 2 Sheets-Sheet (b) swlro/ 12 one/vviding a protective circuit 3,376, 17? Patented Apr. 2, 1968 3,376,477PROTECTIVE CIRCUIT Ralph Weinger, Laverock, Pa., assignor to HarveyHubbell, Incorporated, Bridgeport, Conn, a corporation of ConnecticutContinuation-impart of application Ser. No. 511,923, Dec. 6, 1965. Thisapplication May 12, 1967, Ser. No. 637,942

Claims. (Cl. 317-27) ABSTRACT OF THE DISCLOSU A protective circuit foran alternating current electrical load which includes a resettingsubcircuit. The subcircuit produces a transient current pulse forestablishing a resetting magnetic field regardless of the point in theline voltage cycle at which it is actuated.

This application is a continuation-in-part of my application Ser. No.511,923 for Protective Circuit filed Dec. 6, 1965, now abandoned.

This invention relates to a condition sensing circuit and, inparticular, to a circuit for detecting a change in a predetermineddifferential between two or more currents. The present invention isparticularly useful in pro- Which is easily resettable and inexpensive.

There are many industrial and domestic applications in which it isdesired to provide a circuit which protects against certain circuitfaults or abnormalities. Such abnormalities include current overloads,short circuits, and current leakages to ground. It is also desirable toemploy a device which is effective for protection against major faults,such as heavy overloads and short circuits and against minor faults suchas minor surges, shorts or current leakages. It is also desirable forsuch circuits to include manual means for easily and surely resettingthe circuit once the fault has been corrected. The circuit should alsobe inexpensive to assure a wide market and the distribution of itsbenefits among a large number of people. Among the uses to which such acircuit might be put would be the protection of swimmers in a pool whichis lighted by underwater lights.

It is, therefore; an object of the present invention to provide aprotective circuit responsive to major and minor circuit abnormalitiesthat is less expensive than comparable circuits.

Another object of the invention is to provide an inexpensive protectivecircuit which provides effective protection against major and minorcircuit abnormalities.

Another object of the present invention is to provide an inexpensivecircuit which assures protection against major and minor abnormalitiesand which can easily and surely be reset once the faults have beencorrected.

Other objects, features and advantages of this invention will beapparent to those skilled in the art from the drawings andspecification.

In accordance with the present invention there is provided a protectivecircuit which includes a very sensitive safety switch, and asubstantially less sensitive safety switch. The very sensitive switch isactuated in response to circuit abnormalities to actuate the lesssensitive switch. The less sensitive switch thereupon decouples thecircuit or the device to be protected from the power supply. Anauxiliary subcircuit is employed to reset the very sensitive switch oncethe fault has been corrected. This reset subcircuit enables theresetting function to be accomplished regardless of the point in theline voltage cycle at which it is actuated.

The construction of the invention will now be apparent from thefollowing description, the appended claims and the figures of theattached drawings wherein:

FIG. 1 illustrates the preferred form of circuit embodying the presentinvention;

FIG. 2 illustrates the manner of operation of the circuit of FIG. 1;

FIG. 3 is an illustration of certain 'wave forms for illustrating theoperation of the invention; and

FIG. 4 is an illustration of a modified form of circuit protectivedevice embodying the present invention.

Referring to FIG. 1 there are shown a number of input terminals 1, 2 and3, terminal 1 being connected to ground. There are also a number ofoutput terminals 4, 5 and 6 coupled, respectively, to terminals 1, 2,and 3. A load of any kind may be connected to the output terminals.

Two windings 7 and 8 are shown which are coupled, respectively, betweenterminals 2 and 5 and terminals 3 and 6. They are wound about a unitarycore 9. Although windings 7 and 8 are shown wound about core 9, theycould also pass through the core forming, in effect, single turn primarywindings. A secondary winding 10 is also wound about the core 9. Thewindings 7 and 8 are so wound with respect to the core that, if they aretraversed by equal currents, their respective magnetic fields willcancel one another. Should there be any imbalance in the currentstraversing windings 7 and 8 the resultant magnetic field in core 9 willinduce a current in detector coil 10 which will also flow throughwinding 13. Winding 13 is placed in proximity to a very sensitive reedswitch 12 that is actuated by a very small magnetic field. Also placedin proximity to switch 12 is a latching magnet 21. Magnet 21 establishesa biasing magnetic field which tends to close the reed switch 12 andthereby increases its sensitivity to the magnetic field produced bywinding 13. The magnet 21 also serves to latch the reed switch in theclosed position once it has been actuated by the winding 13. Connectedin series with the Winding 7 between terminals 2 and 5 is a normallyclosed circuit breaker switch 11. The switch 11 is actuated by asolenoid 15 which is connected in series with a current limitingresistor 16 and the reed switch 12 across the load terminals 5 and 6.

The operation of the circuit protective portion of the invention whichhas been so far described may best be understood by reference to FIG. 2in conjunction with FIG. 1. FIG. 2a illustrates the magnetic field inthe vicinity of reed switch 12, and FIG. 2b illustrates the closed oropened state of switch 12. The upper horizontal line of FIG. 2a labeledTrip Level indicates the value of the magnetic field in the vicinity ofswitch 12 which is required totrip switch 12 to the closed position. Thehorizontal line labeled Bias Magnet 21 indicates the steady statemagnetic filed produced in the vicinity of switch 12 by the presence ofthe bias magnet. The lowermost horizontal line labeled Release Levelindicates the lowest value of the magnetic field which will retainswitch 12 in its closed state once it has been tripped. The solid linein FIG. 2a indicates the total resultant field acting upon the reedswitch 12.

Assuming no abnormalities in the load circuit, the only magnetic fieldoperating on the reed switch contacts will be that illustrated at 25produced solely by the bias magnet 21. Assume now that a fault, such asa leakage to ground, develops on the load side of the FIG. 1 circuit.Under these conditions, part of the return current that would normallyflow, say from terminal 6 to terminal 3 through winding 8, will insteadreturn via the external ground. This will produce imbalance between thecurrents in windings 7 and 8 and the resultant magnetic field in core 9will induce a current in detector coil 10 and winding 13. Winding 13will thereupon produce a magnetic field having a value dependent uponthe ampere turns produced by the fault. This magnetic field will besuperimposed upon that produced by bias magnet 21 causing the cyclicalfluctuation indicated in FIG. 2a. When this cyclical fluctuation exceedsthe trip level at point 26, it will actuate the reed switch 12 to itsclosed position as shown in FIG. 2b. The contacts of switch 12 willthereafter remain closed due to the latching function of the bias magnet21. When reed switch 12 closes, it establishes a current path throughwinding 7, switch 11, solenoid 15, resistor 16, switch 12, and winding8. The current traversing solenoid 15 causes it to open the switch 11and thereby cut off power to the load terminals 5 and 6.

Once the circuit fault has been corrected, the protective circuit mustbe reset. In many protective circuits this can be accomplished bymanually closing switch 11. However, since the bias magnet 21 haslatched the contacts of reed switch 12 olosed, closure of the switch 11would merely cause the protective circuit to operate again, reopeningthe switch 11.

In some prior art circuits, the reset circuit included a seriescombination of a diode and a reset winding positioned near the reedswitch for establishing a magnetic field of suflicient magnitude to openthe reed switch. The disadvantage of this arrangement was that if thereset mechanism was actuated at a time when the AC input voltage was notof the proper polarity, the protective circuit would trip once more. Itwould then be necessary to depress the reset button one or more times inthe hope that it would operate during the proper half cycle of the inputvoltage.

In accordance with my invention, I provide a reset subcircuit whichoperates to insure resetting of the protective circuit regardless of thepoint in the input voltage cycle in which resetting is attempted.Furthermore, the reset and circuit breaker switch can be a simple on-offswitch which considerably reduces the expense of the circuit. My novelreset subcircuit comprises an auxiliary winding 14 located near the reedswitch 12, the winding 14 being connected to the output of a full waverectifier bridge 18 through a capacitor 22. Connected across the outputof the bridge and in parallel with the capacitor 22 and auxiliarywinding 14 is a resistor 19. The input to the bridge is supplied fromload terminals 5 and 6 through the current limiting resistor 17.

In order to understand the operation of the reset subcircuit of thisinvention, attention is directed to FIGS. 1, 2 and 3. It should first beunderstood that the objective of this subcircuit is to reduce themagnetic field operating on switch 12 to a value below the release levelindicated in FIG. 2a. In order to do this, it is necessary to provide asufiicient number of ampere turns to accomplish the desired result. Asthe number of turns of the auxiliary winding 14 is fixed, this meansthat current must be introduced in the appropriate amount. Furthermore,this current should be transient in nature so that as soon as thecontacts of switch 12 open they will be ready for subsequent operation.With this understanding, let it be assumed that the circuit fault hasbeen corrected and switch 11 is manually closed during the positive halfcycle of the AC line voltage. A current path will be established throughresistor 17, diode 18a, resistor 19 and diode 18d. The potential thusestablished across resistor 19 will cause a current pulse to passthrough capacitor 22 and auxiliary Winding 14. The magnetic fieldproduced by this current pulse through winding 14 opposes that createdby the bias magnet 21, causing the resultant magnetic field asillustrated in FIG. 2a to be reduced as at 27. As the resultant fielddrops below the release level, the natural resilience of the reed switchcontacts of switch 12 causes them to open. Alternatively, the resultantfield may pass through 0 and create a positive opening effect on thecontacts of reed switch 12. The solenoid and switch .11,'havingarelatively high inertia and inductance, cannot respond to current asrapidly as the reed switch. Accordingly when switch 12 is reopened theresponse of the switch 11 is nullified so that it will not reopen. Atypical response time for switch 12 would be on the order of onemillisecond while that of the circuit breaker formed by switch 11 andsolenoid 15 might be more than several milliseconds. It will be notedfrom FIG. 2a that, after the low point of the magnetic field is reached,it thereafter is reestablished in a rapid exponential manner to thelevel established by bias magnet 21. The particular form of theexponential decay of the current pulse is established by the timeconstant determined by the value of the capacitor 22 and the values ofthe resistors 17, 19.

The manner in which the resetting current pulse is established is shownin more detail in FIG 3.

FIG. 3a illustrates the sinusoidal wave form of the power supply fromthe time 27 at which switch 11 is closed.

FIG. 3b illustrates the train of pulses 28a-28f which are producedacross the output of the full wave rectifier bridge 18. The envelope 29thus represents the current flow passing through capacitor 22 to thereset winding 14.

It will be understood by those skilled in the art that if switch 11 isclosed manually during the negative half cycle of the input voltage thatthe input current to bridge 18 will then pass through diode 18c,resistor 19, diode 18b and resistor 17. However, the voltage establishedacross resistor 19 will have the same polarity as in the previousinstance so that the resultant current pulse and magnetic fieldestablished will be in the same direction and will reopen the switch 12in the same manner.

The reset circuit of this invention can be employed at any time withcertainty and repeated attempts are unnecessary. It will also beapparent that predetermined imbalance in the currents through windings 7and 8 will actuate the protective circuit regardless of the cause ofsuch imbalance. Among the causes of such imbalance would be a leakage ofcurrent from one line to ground. Such a leakage might be established,for example, through a person or animal between either supply lead andground. Regardless of the cause, the protective circuit would operateimmediately. In the form of the invention shown in FIG. 1 the followingcomponents and values were employed and were found to produce highlysatisfactory results although, of course, other values may be chosen forparticular design considerations.

Component No.: Value or description 11, 15 Heinemann P8 circuit breaker.12 Gordos 306 reed switch. 13 72 turns of #20 wire. 14 1000 turns of #38wire. 16 270 ohms, 2 watts. 17 750ohms, /2 watt. 18a, 18b All diodestype 1N645. 18c, 18d Do. 19 220,000 ohms, /2 watt. 22 2 microfarads, 200volts.

In FIG. 4 there is illustrated a modification of the circuit of FIG. 1which is similar in most respects and wherein similar reference numeralsare given to similar elements. The primary distinction is that thewindings 7 and 8 are wound on separate cores 30, 31 and each core is, inturn, provided with a separate secondary winding 32, 33. The secondarywindings are inserieswith each other and with the winding 13. In normaloperation, equal and opposite currents are induced in the windings 32,33 so that no magnetic field is produced by winding 13. Upon animbalance, however, the resultant current sets up the necessary trippingfield adjacent winding 13. The circuit of FIG 1 is,"however, to bepreferred to the circuit of FIG. 4 because of the practical difficultyin achieving proper balance between the cores 30, 31 and theirrespective secondary windings.

It will be understood by those skilled in the art that various othermodifications of this invention are possible. Accordingly, the foregoingdescription is to be construed as illustrative only rather thanlimiting. This invention is limited only by the scope of the followingclaims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a protective circuit which includes a first switch and a secondnormally closed switch coupled thereto which is considerably lesssensitive than said first switch and also includes inductive means fordetecting a predetermined inequality in the currents flowing into andaway from a load circuit to be protected, and in which said inductivemeans actuates said first switch in response to said detected inequalityand said first switch thereupon causes said second switch to open todecouple said load circuit from its source of AC power, means forenabling the resetting of said first switch in response to the reclosureof said second switch after said inequality ceases regardless of thepoint in the cycle of said AC power at which said reclosure occurs.

2. In the protective circuit according to claim 1, wherein saidresetting means includes a full-wave rectifier coupled across said firstand second switches and also includes an inductive means coupled to saidrectifier and positioned near said first switch to open said firstswitch in response to restoration of power to said load by reclosure ofsaid second switch.

3. A protective system for a load circuit comprising:

(a) first switching means which includes latching means,

(b) second switching means coupled to said first switching means, tosaid load circuit, and adapted to be coupled to an input source ofpower, said second means being considerably less sensitive and requiringmore time to respond than said first means,

() means which responds to the detection of a circuit abnormality byactuating said first switching means, said first switching meansthereupon latching and causing said second switching means to openthereby decoupling said load circuit from said input source, and

(d) mean coupled to said input circuit for unlatching said firstswitching means at any desired time in response to the reclosing of saidsecond switching means, said unlatching means operating before saidsecond switching means is capable of reopening in response to thereapplication of power from said source thereto, the unlatching of saidfirst switching means thereby de-energizing said second switching meansand preventing its reopening.

4. The protective system according to claim 3 wherein said (c) meansincludes first and second windings coupled between said input source andsaid load circuit, said first and second windings producing unequal andopposite magnetic fields whenever, due to said circuit abnormality, therespective currents traversing them are unequal by a predeterminedamount and also includes means responsive to said unequal fields, forapplying the differential field to actuate said first switching means.

5. A protective system for a load circuit comprising:

(a) a pair of input terminals adapted to be connected to a source ofpower,

(b) a pair of output terminals adapted to be connected to said load,

(c) first inductive means for producing a first magnetic field coupledbetween one of said input terminals and a corresponding one of saidoutput terminals,

(d) second inductive means for producing a second magnetic field coupledbetween the other of said input terminals and the other of said outputterminals,

(e) a first switch includes latching means therefor,

(f) a second switch coupled to said first switch, said second switchbeing considerably less sensitive and requiring more time to respondthan said first switch,

(g) third inductive means for activating said first switch in responseto a predetermined difference between said magnetic fields, saidactivated first switch thereupon actuating said second switch whichopens to decouple said output terminals from said power source, and

(h) means in circuit across said output terminals for unlatching saidfirst switch at any time after correction of said abnormality, and afterpower has been restored to said output terminals by reclosing of saidsecond switch.

6. The system according to claim 5 wherein said first switch hasmagnetic latching means and wherein said resetting means includes afull-wave rectifier bridge whose input is coupled to said outputterminals and whose output is coupled to a fourth inductive meanslocated in proximity to said first switch, said fourth inductive meansproducing a magnetic field in response to the restoration of power tosaid output terminals which overcomes the effect of said magneticlatching means.

7. The system according to claim 5 wherein said first switch is a reedswitch and said third inductive means includes a first windingpositioned to be threaded by said first and second magnetic fields andalso includes a second winding in circuit with said first winding andpositioned near said reed switch for closing the latter whenever apredetermined voltage is induced in said first winding by the differencein said first and second magnetic fields.

8. The system according to claim 6 wherein said resetting means alsoincludes a capacitor between the output of said bridge and said fourthinductive means.

9. The protective system according to claim 5 wherein said power sourcefurnishes alternating current, wherein said second switch includes apair of normally closed contacts between said other input terminal andsaid other output terminal, said second switch also including a solenoidfor opening said contacts when energized, said solenoid having oneterminal coupled to one of said contacts, and wherein said first switchhas one terminal coupled to the other terminal of said second switch andits other terminal coupled to said other output terminal.

lit. In a protective circuit including means for generating a trippingsignal in response to a predetermined inequality in the currents flowinginto and away from an alternating current load circuit to be protected,circuit breaker means in said load circuit, sensitive switch meansclosable in response to said tripping signal to open said circuitbreaker means, and means for latching said sensitive switch means in itsclosed position, the improvement which comprises: a resetting windingadjacent said sensitive switch means to establish a magnetic fieldacting to open said sensitive switch means; and current pulse producingmeans energizable from said load circuit upon closure of said circuitbreaker means to supply a unidirectional transient current to saidresetting winding regardless of the polarity of the load voltage at thetime of closure.

References Cited UNITED STATES PATENTS 3,165,671 1/1965 M'intz et al317-27 3,214,638 10/1965 Moser et al 317-27 X 3,337,771 8/1967 Weinger31727 X MILTON O. HIRSHFIELD, Primary Examiner. J. D. TRAMMELL,Assistant Examiner.

